Proton exchange membrane fuel cells:
PEM fuel cells are widely popular fuel cells which find application mainly in the automotive sector, where they are expected one day to replace gasoline and diesel internal combustion engines, and in the domestic sector, for example, as replacements for rechargeable batteries.
PEMFCs are typically fuelled by hydrogen molecules and comprise a solid proton exchange membrane, in the form of a thin film (usually 125-180 μm thick) sandwiched between an anode and a cathode (see FIG. 1). The hydrogen molecules are fed at the anode, where they are split into hydrogen ions (H+) and electrons (e−) by a platinum catalyst. The membrane lets through H+ ions and, by way of its electrically insulating nature, leaves behind the electrons, which create an electron flow through an external circuit, and produce electric power and heat. At the opposite end, the cathode receives the hydrogen ions exiting the membrane, the electrons (which have traveled through the external circuit) and the oxygen (provided by the surrounding air), thus producing water, which flows out of the cell. The only waste product is water vapor and/or liquid water.
The entire process can thus be summarized by way of the following reactions:Anode: 2H2=>4H++4e−Cathode: O2+4H++4e−=>2H2OOverall cell reaction: 2H2+O2=>2H2O
PEM fuel cells are suitable for many applications in which a light, compact, and relatively inexpensive power source is required. As water is the only liquid present in the cell, the typical operational temperature is around 80° C. (60-100° C.), which makes it ideal for those applications which require rapid start-up. Given such relatively low operational temperature, the use of catalysts (usually platinum-based) is mandatory.
These cells are typically associated with an efficiency in the order of 40-50% and the production of about 0.8 volts. Cells can be arranged in series or in parallel to achieve the desired voltage. To run a car, for example, about 300 volts are required. Electrical power in the order of 50 kW has been achieved and units producing up to 250 kW are currently under development.
Although they are efficient and convenient under most practical circumstances, PEMFCs do suffer from a number of drawbacks. Firstly, the amount of water evaporating must always be lower than the amount being generated, as the cell cannot be allowed to dry out. Moisture control is thus essential.
Secondly, due to the low temperature of operation (imposed by the water-based nature of the cell), there is the need for high quantities of highly sensitive catalysts, which can be expensive.
Moreover, the sensitive catalysts require pure hydrogen as fuels meaning that the cell must be free of impurities.
The operational limit of 100° C. is not ideal as it is not usually a high enough temperature to perform useful co-generation. Finally, the membrane materials can be very expensive.
There is, therefore, a strong-felt need in the field for the production of proton electrolyte membranes capable of operating at temperatures higher than 100° C. and of PEMFCs capable of supporting more tolerant catalyst systems.
Solid electrolyte membranes suitable for PEMFCs are conventionally made of solid organic perfluorinated polymers, such as, for example poly-perfluorosulphonic acid.
One such membrane, which is commercially available and widespread, is Nafion™ (by DuPont), a perfluorinated polymer with side chains terminating in sulfonic acid moieties. With a membrane thickness in the range of 50 to 175 μm, they exhibit high proton conductivity, high chemical stability under typical operating conditions and low gas permeability. Disadvantages typical of these membranes include their high synthesis costs, due to the fluorinated monomers, low proton conductivity at low water content, low mechanical resistance at temperatures above 100° C., and finally, membrane crossover problems.
Styrene derivatives, consisting of a fluorinated monomer chain backbone side-linked to styrenic rings, are also quite popular choices for the manufacture of proton electrolyte membranes. Such membranes, however, exhibit very low oxidative stability.
Recently, in the effort to overcome the limits of the existing membranes, research has focused on the development of membranes based on sulfonated polyether polymers.
U.S. Pat. No. 6,869,980, recently granted to Cui (Mar. 22, 2005) discloses novel polymer blend membranes comprising a functional polymer based on sulfonated aryl polymers, a reinforcing polymer based on aminated or nitrated polyether sulfones and/or polyether ether sulfone and a plasticizer as PVDF.
U.S. Pat. No. 6,914,084 B1 (Jul. 5, 2005, to Soczka-Guth et al.) discloses fuel cell membranes comprising sulfonated polyether ketone and another polymer, a process for their production and their use. The membrane contains: sulfonated, strictly alternating, polyether ketones (A) (30-99.5% by weight) with repeating units of formula (I): —Ar—O—Ar′—CO, (I), where Ar and Ar′ are, independently of one another, bivalent aromatic radicals, with ion exchange capacity comprised between 1.3 and 4.0 meq. (—SO3H)/g (polymer) and from 0.5 to 70% by weight of a fluorinated, nonfluorinated or perfluorinated polymer (B).
European Patent No. 0574791 (also published as U.S. Pat. No. 5,438,082) to Helmer-Metzmann discloses the manufacture of a polymer electrolyte membrane from sulfonated, aromatic polyether ketone, an aromatic polyether ketone of the formula (I)
in which Ar is a phenylene ring having p- and/or m-bonds, Ar′ is a phenylene, naphthylene, biphenylene, anthrylene or another divalent aromatic unit, X, N and M, independently of one another are 0 or 1, Y is 0, 1, 2 or 3, P is 1, 2, 3 or 4, is sulfonated and the sulfonic acid is isolated. At least 5% of the sulfonic groups in the sulfonic acid are converted into sulfonyl chloride groups, and these are reacted with an amine containing at least one crosslinkable substituent or a further functional group, and unreacted sulfonyl chloride groups are subsequently hydrolyzed. The resultant aromatic sulfonamide is isolated and dissolved in an organic solvent, the solution is converted into a film, and the crosslinkable substituents in the film are then crosslinked. In specific cases, the crosslinkable substituents can be omitted. In this case, sulfonated polyether ketone is converted into a film from solution.
The technical problem underlying the present invention is thus that of providing a novel proton electrolyte membrane suitable for use in fuel cells, which exhibits high proton conductivity (higher than 10−2 S/cm) while maintaining a low electronic conductivity, a low permeability to fuel and oxidant (lower than 10−7 cm2/s), oxidative, thermal and hydrolytic stability at temperatures higher than 100° C., low water uptake (lower than 30%) and finally, good mechanical properties in both the dry and hydrated state.